1. Field of the Invention
The invention relates in general to a storage system for storing data for use by host computers, and in particular, to a cache residency setting technique for the storage system.
2. Description of Related Art
Storage apparatuses are connected via networks to host computers, thereby forming a computer system. One configuration for orchestrating a plurality of storage apparatuses is an external storage connection configuration where a first storage apparatus connects a plurality of second storage apparatuses while it is connected to host computers. In this configuration, the first storage apparatus logically integrates its own logical devices with the logical devices in the second storage apparatuses and thereby provides them as a whole to the host computers, allowing the host computers to I/O-access the first storage apparatus without being aware of the existence of the second storage apparatuses.
For enhanced system performance, storage apparatuses usually have mass storage cache memory devices for providing cache functions. With a typical cache function, in terms of effective use of the cache memory, the data in the cache memory is replaced in accordance with an LRU algorithm or similar. In addition, the storage apparatuses have cache residency functions for having the data reside in predetermined areas in their cache memories at all times, while taking the characteristics of the application data into consideration.
JP2005-11277 A discloses a computer system that employs an external storage connection configuration where a storage is connected to a plurality of external storages. In this computer system, when the storages have disk cache functions, such as cache residency etc., being set, they perform input/output processing via their cache memories.
JP1998-63576 A discloses a disk array controller composed of an upper layer controller and lower layer controller, each having a cache memory. This disk array controller executes control so as not to store duplicated data in such hierarchical caches.
In storage systems adopting external storage connection configurations, external storage apparatuses—second storage apparatuses—individually manage data based on logical addresses unique only in their own storage apparatuses; accordingly, it has been extremely troublesome to set cache residency for these second storage apparatuses, which can be viewed via the first storage apparatus.
Moreover, even if cache residency is set for the second storage apparatuses, the data targeting the second storage apparatuses is also cached in the cache memory in the first storage apparatus, and thus the areas in the cache memory in the first storage apparatus are consumed more than necessary. In other words, the more the access to the second storage apparatuses increases, the more the areas in the first storage apparatus's cache memory are consumed, and the first storage apparatus cannot ensure sufficient areas in its cache memory for its own use.